Many types of articles require cleaning or removal of coatings. Removing hardened coatings such as paint from a surface can be difficult, time consuming, and costly. Moreover, certain removal methods generate significant amounts of environmentally hazardous wastes.
One example is the removal of paint from the surface of aircraft. Military aircraft especially have need for periodic removal of worn or damaged paint. Military aircraft are painted for purposes of camouflage, structural integrity, reduction of radar reflectivity, etc. Once an aircraft is painted, it must be periodically stripped and repainted as new paints become available or as old paint coats become worn or damaged. Known paint stripping methods are time consuming and expensive. Because known paint removal processes take so much time, the aircraft is either out of commission for a lengthy period of time or must be flown with a compromised coating.
Three methods are currently in use for removing paint from the surface of aircraft. A first method uses chemical solvents, typically petroleum-based, to remove the paint from the aircraft surface. The chemical method produces approximately 17,000 pounds of hazardous liquid waste byproducts for a military fighter aircraft such as an F-15. Use of chemical solvents requires protective clothing and respirators during the stripping process, and procedures for collecting and storing the hazardous waste byproducts. Such measures significantly increase the cost of the process and are a threat to the quality of the environment.
A second known paint removal method involves blasting with media such as plastic pellets. The plastic pellets are blasted with high pressure air onto the surface to be cleaned. The impact of the pellets causes the physical removal of the paint by abrasion. As with chemical solvents, use of plastic pellet blasting require the use of protective clothing and respirators. This process produces significant amounts of waste in the form of plastic pellets mixed in with paint chips. The collection and disposal of the plastic pellets and paint chips is time consuming.
A third paint removal method physically removes paint from the surface of an aircraft using media comprising particles of solid CO.sub.2 instead of plastic. The carbon dioxide or "dry ice" is formed into solid pellets and propelled at high speed onto the surface of the aircraft. The impact of the dry ice pellet chips the paint off the aircraft surface. At ambient temperatures and pressures, the CO.sub.2 pellets evaporate into the atmosphere through a process known as sublimation. After the CO.sub.2 pellets sublimate, the only remaining by-products of this paint removal process are dried paint chips.
A CO.sub.2 pellet removal system produces about 240 pounds of dried paint chips for an F-15 aircraft. In comparison to the chemical stripping process, the CO2 process produces 70 times less waste. Advantageously, the dried paint chips are easier to gather up and confine, and the process does not produce environmentally hazardous byproducts that must be separately handled and disposed. Moreover, methods of paint removal involving use of CO.sub.2 pellets are considered environmentally benign since the CO.sub.2 is usually obtained as a byproduct of other industrial processes, and no new CO.sub.2 is introduced into the atmosphere.
Present CO.sub.2 paint removal systems typically include a liquid CO.sub.2 storage tank, a pelletizer that converts CO.sub.2 from liquid into solid pellet form, a feeder that feeds the CO.sub.2 pellets into a stream of high pressure air, a conduit or hose connected to the feeder, and a nozzle. One example of a paint removal system and method that employs CO.sub.2 pellets as particulate media is described in U.S. Pat. No. 5,109,636 to Lloyd et at., entitled "Particle Blast Cleaning Apparatus and Method".
Known CO.sub.2 paint removal systems have certain drawbacks. For example, most known systems are inefficient since only a small portion of the CO.sub.2 pellets produced by the pelletizer are ultimately ejected from the nozzle. Some of the inefficiency is believed due to the temperature difference between the nozzle and point at which the pellets are introduced into the pressurized air stream, which can be several degrees. Moreover, the temperature of the pressurized air stream into which the pellets are introduced is typically many degrees warmer than the temperature of the CO.sub.2 pellets.
The present inventors have discovered that only about 18% of the pellets actually produced by the pelletizer in known prior art systems ultimately reach the aircraft surface. It it believed that a substantial portion of the CO.sub.2 pellets sublimate prior to exiting the blast nozzle. The number of pellets that actually impact the surface to be cleaned affect the overall efficiency of the system and the rate of cleaning.
Known CO.sub.2 paint removal systems also cause undesirable backside paint removal under certain conditions. Aircraft surfaces having a thickness of 0.040 inches or less sometime experience partial removal of paint from the opposite or "backside" of the surface. The problem seems to occur most often when larger size pellets are used (greater than 0.080 inches in diameter), which apparently impact the surface with such force that a localized deflection is created that passes through the metal to the back side of the surface, where it contributes to removal of paint from that surface.
Accordingly, there is a need for an improved CO.sub.2 pellet cleaning system that is more efficient in terms of pellet survival and provides better cleaning ability than prior art systems. In this regard, there is a need for an improved CO.sub.2 pellet cleaning method and system that reduces sublimation of the CO.sub.2 pellets before they are propelled from the nozzle.